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Mitochondrial Genomes Reveal a Close Relationship Between Two Hawk-Eagles

Indiaskogørn (Nisaetus cirrhatus) på en grein i Sundarbans nasjonalpark, India
Research · Genetics & genomics

What can mitochondrial DNA reveal about the relationships among hawk-eagles? Researchers reported near-complete mitochondrial genomes for the South Philippine Hawk-eagle (Nisaetus pinskeri) and the Changeable Hawk-Eagle (Nisaetus cirrhatus), adding genetic reference data for two species with limited genomic information. A mitochondrial genome is the DNA found in a cell’s mitochondria, separate from its nuclear genome. For some Southeast Asian Nisaetus hawk-eagles, limited ranges and fragmented distributions have made it difficult to characterize these birds.

The reported sequences were 18,654 base pairs long for the South Philippine Hawk-eagle and 17,874 for the Changeable Hawk-Eagle. Both contained an extra cytosine—a DNA building block—at position 174 in the mitochondrial gene nad3. This gene encodes part of complex I in the mitochondrial electron transport chain, which is central to cellular energy production. In principle, the extra DNA base shifts the reading frame, changing how the genetic code is divided into instructions for making a protein. Similar insertions are known in other birds. The researchers describe two proposed ways a functional protein might still be produced: a shift in the reading frame during translation or RNA editing. The study does not establish which mechanism, if either, operates in these hawk-eagles. The insertion had not previously been reported in the other sampled Nisaetus species.

The two species form a close pair in the mitochondrial family tree

Comparisons of mitochondrial sequences placed the South Philippine Hawk-eagle and Changeable Hawk-Eagle close together, as sister to the other sampled Nisaetus representatives—that is, the two species form a pair on a branch closest to the branch containing the others. Trees combining information from several mitochondrial protein-coding genes were more similar to recently reported phylogenies than trees based on individual, commonly used mitochondrial markers. Using several genes together therefore produced a picture of their relationships that better matched those recent evolutionary analyses.

Genes involved in energy production evolve at different rates

Mitochondrial genes encode parts of the molecular machinery that converts energy from nutrients into ATP, the cell’s main energy carrier. cox1 encodes a component of cytochrome c oxidase, the final enzyme complex in the electron transport chain. atp8 encodes a component of ATP synthase, the enzyme complex that uses the energy stored in a proton gradient to make ATP.

The researchers also examined how strongly these genes were affected by purifying selection. By comparing DNA changes that alter a protein’s amino acids with changes that do not, scientists can assess how strongly evolution has acted against changes to the protein. All the examined genes showed purifying selection, but it was strongest for cox1 and weakest for atp8.

The amount of DNA variation also differed: 60.5% of positions in atp8 varied among the studied raptors, compared with 35.9% in cox1. The authors note that atp8 is a short gene, which may contribute to its high proportion of variable positions. In contrast, cox1 is sufficiently conserved across species to be widely used as a DNA barcode for identifying them. The study does not establish a specific molecular cause of the difference in selection pressure between these genes.

The new sequences expand the genetic resources available for understudied hawk-eagles, including the endemic and endangered South Philippine Hawk-eagle. They provide a foundation for future research into genetic variation within populations and evolutionary relationships among these birds.

Sources and image credit

  1. The mitochondrial genomes of the endemic and endangered South Philippine Hawk-eagle Nisaetus pinskeri and other Nisaetus Hawk-eagles (Aves, Accipitriformes)

Image: Changeable Hawk-Eagle (Nisaetus cirrhatus) photographed in Sundarbans National Park, India, in July 2025. Representative photograph. Photo: Tisha Mukherjee / Wikimedia Commons · CC BY-SA 4.0 · Source

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